A number of countries in Asia and in Europe currently require or are expected to require biodegradable solid waste and food waste solids from residences, as well as restaurants and institutions that prepare and serve food, to be source segregated from metal, glass, plastics, paper, and other solid waste materials. This regulatory trend is likely to be adopted by larger cities in both the United States and Canada. Currently, disposal of food waste-solids in these countries is by landfill, incineration and to a limited degree composing to produce soil amendment.
South Korea is one representative country seeking to restrict disposal of food waste and biodegradable solid waste. The Korean national government has indicated it intends to prohibit landfill disposal of food wastes by Jan. 1, 2006, and ocean dumping of wastewater treatment biosolids by Jan. 1, 2004. Food wastes are composed primarily of vegetables, fruits, grains, and meat/fish products that contain fiber, carbohydrate, fat, protein and inorganic ash. Data indicates that Korean source segregated food wastes contain 76-84% moisture and 16-24% dry matter. Additionally, the dry matter is 80-90% volatile. Incineration is a costly alternative, skeptically viewed by the Korean public because of environmental concerns.
As an alternative to conventional landfill or incineration disposal, particulate food wastes, wastewater treatment biosolids, and other biodegradable organic particulate matter (i.e. manure, green wastes, source separated biodegradable municipal solid waste, etc.) can be first solubilized by thermal hydrolysis, with the liquid fraction converted, under suitable anaerobic conditions to methane gas and carbon dioxide. The residual carbonaceous material containing nitrogen and phosphorus is suitable for use as a soil amendment or as a fertilizer with further nitrogen and phosphorus fortification. Alternatively, the residual carbonaceous material has sufficient caloric value so as to be suitable for use as a source of fuel to generate energy.
Thus, thermal hydrolysis of particulate biodegradable organic wastes followed by anaerobic conversion of the solubilized fraction of these organic solid wastes potentially provides an economically attractive and environmentally sound disposal alternative offering significant benefits.
Yokoyama, U.S. Pat. No. 5,707,417, is directed to a method for treating garbage including thermal hydrolysis followed by anaerobic digestion. More particularly, Yokoyama is concerned with the treatment of garbage resulting from the preparation, cooking and dispensing of food containing water-insoluble organic compounds. Yokoyama teaches thermal hydrolysis of food waste for periods of 1-720 minutes at temperatures between 100-400° C. (212-752° F.) and at pressures higher than the corresponding water vapor saturation pressure (2-200 atm or 29.4-2940 psig). Following solubilization, the solubilized organics and insoluble particulates are anaerobically digested to produce methane. Yokoyama further teaches that the soluble and insoluble products from thermal hydrolysis may be separated before subjecting the liquid phases to anaerobic digestion. Yokoyama then teaches that the solid phases can be recovered and burned or processed into compost. While Yokoyama provides considerable benefit over conventional landfill or incineration disposal of food wastes, the process taught by Yokoyama is not energy efficient, nor does it maximize solubilization or liquid removal from solid phases. Further, Yokoyama does not minimize the soluble fraction recalcitrant to biodegradation or ammonia produced from thermal hydrolysis.
Solheim, U.S. Pat. No. 5,888,307, teaches a continuous multi-stage system consisting of four steam reaction vessels operated in series to solubilize organic waste materials, including sewage sludge. Solheim teaches that to minimize heating costs and to conserve energy, flash steam from the fourth stage reaction vessel can be combined with supplemental steam to heat the first stage reaction vessel. In addition, flash steam from the third stage reaction vessel can be used to heat the second stage reaction vessel. Solheim teaches the hydrolysis occurs in a temperature range of 160-240° C. (320-464° F.). Solheim further teaches that acid can be added in order to obtain enhanced hydrolysis. Solheim fails to optimize energy usage in the hydrolysis process. Furthermore, Solheim does not teach any benefit in separating solubilized organics and residual solids before anaerobic digestion.
Shih, U.S. Pat. No. 5,525,229, teaches an apparatus and method for anaerobic digestion of organic materials such as sewage sludge, municipal waste, animal waste and other highly organic materials. Shih discloses bacterial hydrolysis of such waste prior to anaerobic digestion to enhance soluble organics available for anaerobic digestion. Shih also teaches recycling heat from the digestion process to preheat materials being added to the hydrolysis step. The bacterial hydrolysis step taught by Shih fails to provide sufficient solubilization and solids dewatering potential to minimize the volume of residual solids. It, therefore, is not fully effective at minimizing the volume of solid wastes which must be disposed of.
The present invention is intended for overcoming one or more of the problems discussed above.